The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided on an exhaust passage with a NOx catalyst for purifying NOx in exhaust gas.
Conventionally, NOx storage catalysts which store (occlude) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known. Such NOx storage-reduction catalysts further reduce the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). Within a normal operating range of an engine, the engine is operated at the lean air-fuel ratio (λ>1) so as to reduce fuel consumption, although if this lean operation state continues for a while, the NOx stored amount in the NOx catalyst reaches a limit value and the NOx catalyst can no longer store NOx, which causes NOx to be released. For this reason, the air-fuel ratio is suitably set to be stoichiometric or richer (λ≤1) in order to reduce NOx stored in the NOx catalyst. Note that “λ” is an index of the air-fuel ratio expressed with reference to the theoretical air-fuel ratio, and is a so-called air excess ratio.
For example, JP2004-360593A discloses an art for executing, when a NOx stored amount in a NOx catalyst exceeds a given amount, a fuel injection control to enrich an air-fuel ratio of exhaust gas so as to reduce the NOx stored in the NOx catalyst.
One example of a setting method of an air-fuel ratio of exhaust gas so that NOx stored in a NOx catalyst becomes reducible (hereinafter, this air-fuel ratio is referred to as “target air-fuel ratio”) is performing a post injection after a main injection. In the main injection, fuel is injected into a cylinder so as to output a desirable engine torque, and in the post injection, fuel is injected at a timing so that the engine torque output is not influenced (typically, on expansion stroke). The fuel injected in the post injection typically needs to be combusted inside the cylinder in order to perform the reduction of the NOx catalyst, otherwise unburned fuel is discharged and degrades emission performance of HC (hydrocarbon), etc.
In this regard, JP2004-360593A suggests that when reducing NOx stored in the NOx catalyst, an exhaust gas recirculation (EGR) gas amount recirculated back to an intake system is increased and a fresh air amount introduced into the engine is reduced so as to enrich the air-fuel ratio of the exhaust gas. However, increasing the EGR gas amount in the configuration for performing the post injection and combusting the fuel injected in the post injection inside the cylinder during the reduction of the NOx catalyst as described above, may degrade combustion stability and the fuel injected in the post injection may not properly be combusted inside the cylinder. In this case, HC which is unburned fuel is generated.
Although the introduction of EGR gas during the reduction of the NOx catalyst may be prohibited to prevent such HC generation, this makes it impossible to suitably control an oxygen concentration within the cylinder, and smoke (soot) may be generated when the fuel injected in the post injection is combusted inside the cylinder.